The present invention relates to stability in industrial lifting machines and, more particularly, to a measurement system for a lifting vehicle for assessing machine stability.
As a boom is extended and a load is applied to the platform or bucket thereof, the vehicle or lift structure's center of mass moves outwardly toward the supporting wheels, tracks, outriggers or other supporting elements being used. If a sufficient load is applied to the boom, the center of mass will move beyond the wheels or other supporting elements and the vehicle lift will tip over.
In the context of boom lifts, two types of stability are generally addressed, namely “forward” and “backward” stability. “Forward” stability refers to that type of stability addressed when a boom is positioned in a maximally forward position. In most cases, this will result in the boom being substantially horizontal. On the other hand, “backward” stability refers to that type of stability addressed when a boom is positioned in a maximally backward position (at least in terms of the lift angle). This situation occurs when a boom is fully elevated, and the turntable is swung in the direction where the turntable counterweight contributes to a destabilizing moment. In most cases, this will result in the boom being close to vertical, if not completely so.
Typically, not only can a boom be displaced (i.e., pivoted) through a vertical plane, but also through a horizontal plane. In a boom lift, for example, horizontal positioning is usually effected via a turntable that supports the boom. The turntable, and all components propelled by it (including the boom and work platform), are often termed the “superstructure.” As the wheeled chassis found in typical lift arrangements will usually not exhibit complete circumferential symmetry of mass, it will be appreciated that there exist certain circumferential positions of the boom that are more likely to lend themselves to potential instability than others. Thus, in the case of a lift in which the chassis or other main frame does not exhibit symmetry of mass with regard to all possible circumferential positions of the boom, then a greater potential for instability will exist, for example, along a lateral direction of the chassis or main frame, that is, in a direction that is orthogonal to the longitudinal lie of the chassis or main frame (assuming that the “longitudinal” dimension of the chassis or main frame is defined as being longer than the “lateral” dimension of the chassis or main frame). Thus, when incorporating safety requirements into the lift, these circumferential positions of maximum potential instability must be taken into account.
A more detailed discussion of lift machine stability can be found in U.S. Pat. No. 6,098,823, the content of which is hereby incorporated by reference.
Stability problems can also arise due to operator improper operation or misuse, for example, if an operator attempts to lift extra weight and exceeds the machine capacity. When overloaded, the loss of machine stability could lead to the machine tipping over. Improper operation or misuse could also arise if an operator gets the machine stuck in the mud, sand, or snow and proceeds to push himself out by telescoping the boom and pushing into the ground. This also leads, in addition to possible structural damage and malfunctioning of the machine, to a tipping hazard. Still another example of improper operation or misuse could occur if an operator lifts a part of the boom onto a beam or post and continues to try to lift. The result is similar to the overloading case.
The use of stability limiting and warning systems in load bearing vehicles has been practiced for several years. Most have been in the form of envelope control. For example, given the swing angle, boom angle, and boom length, a conservative envelope stability system could be developed for a telescopic boom lift or crane. In this method, the number of sensors necessary to achieve the stability measurement is high and contributes to poor reliability and increased cost, especially for machines with articulating booms. In addition, the load in the platform needs to be independently monitored. Another practiced method is to measure boom angle and lift cylinder pressure. In theory, as the load increases, the pressure in the cylinder supporting the boom also increases. But in reality, it is more complicated. Indeed at high angles, for example, much of the load passes into the boom mounting pins and will not result in an appropriate increase in cylinder pressure. Also, hysterisis errors are significant, where the pressures may substantially differ for the same boom angle depending on whether the boom angle was reached by raising or lowering the boom.
Several other similar methods can also be found on the market. However, similar to the methods described above, they use a large number of sensors and lack the ability to address backward stability situations.